and expressed in a high-density fermentation system, this modified operon produces extremely large quantities of this recombinant urease—over 3 grams per liter of culture, when immunization requires microgram or nanogram doses (see below).
Preclinical Results. There are several good animal models in which to test HP vaccines. H. pilus is a very similar species that infects mice and cats, and the human pathogen H. pylori can also be modified to infect mice. Ferrets are susceptible to a third species. However, the most interesting work has been done with cats, which are highly susceptible to H. pylori, naturally infected, and develop gastritis and even ulcer disease that is very similar to humans. Cat immunology is at a relatively advanced state of knowledge, which makes this model even more useful. Finally, researchers are beginning to work with primates as well.
Experience has shown that the most effective schedule for prophylactic vaccination is 4 applications 1 week apart. The animals are then tested for antibodies and challenged with either H. pilus or H. pylori after an appropriate interval—initially 2 weeks, but now up to 1 year. A total of 2 weeks or longer after challenge, the animals are sacrificed and tested in various ways: spectrophotometry (to measure urease activity in the stomach and hence the presence of bacteria), histology (a silver stain for bacteria), electromicroscopy (for antibodies), and immunocytochemistry (for antibody-secreting cells in the gastric mucosa).
Experiments with H. pilus in mice showed that mucosal immunity plays an important role in protective immunity. Experimental animals were immunized with recombinant urease by different routes: oral, oral with bicarbonate, intragastric, intragastric with bicarbonate, and subcutaneous. Following challenge, 100 percent of unimmunized controls became infected, while 100 percent of those immunized by the oral route were protected. Intragastric immunization, which bypasses potential induction sites in the oral cavity, was less protective, especially when given without bicarbonate to neutralize gastric acid. Parenteral immunization did not protect at all, although it did induce very high serum IgG antibodies.
Dose-response studies showed this is an incredibly potent immunogen, but that a mucosal adjuvant is required for immunization. When mice were given recombinant urease linked to E. coli labile toxin (LT, similar to the LT-B used as an adjuvant in streptococcus vaccines, above) by the oral or intranasal route, doses as low as 50 nanograms provided significant protection. Doses are usually in the microgram range or higher in animal experiments. Without the mucosal adjuvant, however, doses as high as 5 milligrams provided no protection: 100 percent of the animals became infected. Analysis of antibody response indicate that secretory IgA correlates best with protection: animals that were fully or partially protected had IgA in their saliva; those that became infected might have IgG in their serum but had no IgA in their saliva.